It is known to perform open-loop or closed-loop control on internal combustion engines as a function of a composition of their exhaust gases, wherein the corresponding exhaust gas component is measured by means of a suitable exhaust gas probe. In particular the air/fuel ratio with which the engine is operated is regulated by measuring the oxygen content of the exhaust gas by means of a lambda probe in the exhaust gas section. This procedure is generally referred to as lambda control. In this case, the lambda probe makes available an actual probe signal which is dependent on the oxygen content of the exhaust gas and which is usually a probe voltage. This probe signal is converted into the lambda value by means of a stored characteristic curve or a corresponding calculation rule, and said lambda value is used for the regulating process.
However, conversion of the probe signal into a lambda value is in practice made more difficult by the fact that the probe signal not only depends on the exhaust gas composition but is also influenced by additional interfering influences which have the effect that the characteristic curve is not constant under all conditions. In the case of step-change probes, it is known, for example, that the probe temperature, that is to say the temperature of the measuring element of the probe, has an influence on the accuracy level of the conversion rule or of the characteristic curve. This has an effect, in particular, in the rich lambda range, that is to say at lambda values <1. In addition, changes in the characteristic curve characteristics result through increasing aging of the measuring element of the probe over the service life. Furthermore, various exhaust gas components such as lead, manganese, phosphorus or zinc can cause progressive contamination of the measuring element and therefore a change in the characteristic curve.
DE 100 36 129 A discloses compensating influences of the temperature on the probe signal. For this purpose, the probe temperature is determined as a function of the internal resistance of the probe from a stored characteristic curve. By using a three-dimensional characteristic diagram which maps a correction voltage as a function of a current probe actual voltage and the previously determined probe temperature, the current correction voltage is then determined and added to the current probe actual voltage in order to obtain a corrected probe voltage.
DE 199 19 427 A describes a method for correcting a characteristic curve of a broadband lambda probe which is installed upstream of an exhaust gas catalytic converter, wherein in an overrun shutoff phase of the internal combustion engine the sensor signal of the lambda probe is evaluated and the signal level which is determined in this way is used for the correction of the gradient of the characteristic curve.
DE 10 2007 015 362 A discloses a method for calibrating a step-change lambda probe which is arranged upstream of a catalytic converter. For this purpose, a correction signal is determined from a measurement signal which is made available by a reference lambda probe connected downstream, and said correction signal is used to adapt the characteristic curve of the step-change lambda probe.
It is disadvantageous with all the known methods that the corrections also only have a limited accuracy level and therefore deviations of the corrected characteristic curve from the exact characteristic curve can remain. This fact is taken into consideration in the prior art by defining lambda setpoint values or lambda threshold values which are to be adjusted and whose attainment triggers a change in the air/fuel mixture with a safety interval which takes into account the uncertainty. This safety interval is usually dimensioned in such a way that the largest degree of inaccuracy to be assumed for the characteristic curve is also taken into account.
A typical example of this procedure is the enrichment of an engine, which enrichment is performed to protect components against overheating. In this context, the combustion temperature and therefore the exhaust gas temperature are lowered by feeding in additional fuel, and overheating, for example of turbochargers or catalytic converters, is therefore prevented. The enrichment of the mixture in order to protect components is usually carried out when a permissible limiting temperature, for example of 900° C., is reached, wherein a target lambda value of for example 0.9 is set by feeding in additional fuel, said value ensuring an effective cooling effect. If, for example, a maximum tolerance range of 2% is allowed for when using the lambda probe, a lambda threshold of 0.88 is conventionally predefined for the engine in order to remain reliably under the necessary limit of lambda 0.9 under all conditions. However, in the majority of engines which have a lambda probe with relatively small tolerance deviation, this results in a relatively large amount of enrichment and consequently in a higher consumption of fuel than would be actually necessary.